Further extension of earlier studies on separation and evaluation of the biologic potential of the various antigens of M. leprae have led to the identification of a fraction that possesses a single antigen, in addition to other fractions that possess more than one antigen. Hypersensitivity studies on these fractions have indicated that the fraction containing the single antigen is more specific in terms of elicitating skin reactivity, whereas the other fractions have shown cross reactions in animals sensitized with other mycobacteria. These observations have been further confirmed by passive cutaneous anaphylaxis studies. In addition to these studies, efforts were made to identify by various immunologic tests the acid-fast organism isolated from infected armadillos. These studies have confirmed that this organism is M. leprae. Studies have also been conducted on assaying cell mediated immune response in mice infected with M. leprae by foot-pad method. The Jerne plaque assay method has shown that there is a very early immune response in these animals despite the fact that the actual infection progresses at a very slow pace. Appropriate controls were used to determine whether the response observed was not originated through means other than the absolute infection, a possibility that cannot be overlooked in such infections.